1. Field of the Invention
The present invention relates to a coating applied onto a substrate and a method for applying the coating. The invention specifically described is an improved coating for optical fibers. The coating consists of a densely packed structure of sputtered particles forming a precise, dense, and adherent layer. The coating is deposited within a cylindrical magnetron via a sputtering process that avoids damaging the optical fiber.
2. Description of the Related Art
In recent years, optical fiber technology has gained popularity in many commercial applications due to unparalleled performance advantages over existing metal-wire systems. In particular, optical fibers and related components are widely accepted in military communications, civilian telecommunications, and control systems. Optical fibers are small, strong, and lightweight. In communication applications, they provide wide bandwidth, low transmission loss, resistance to radiation damage, and immunity to electromagnetic interference.
A typical optical fiber is composed of a core within a layer of cladding and thereafter one or more layers of a buffer. The core provides a pathway for light. The cladding confines light to the core. The buffer provides mechanical and environmental protection for both core and cladding.
Fiber construction and materials are known within the art. For example, a typical single-mode fiber (SMF) is composed of precision extruded glass having a cladding with a diameter of 125 xcexcmxc2x12 xcexcm and a core with a diameter of 8 xcexcmxc2x11 xcexcm residing within the center of the cladding. A buffer is typically composed of a flexible polymer applied onto the outer surface of a cladding via known methods yielding dimensional variations at least one magnitude larger than in core and cladding. Existing deposition methods produce a coating with large dimensional variations. Consequently, state-of-the-art optical fibers are composed of a dimensionally precise core and cladding assembly within a less precise buffer and coating. Such imprecisions skew the concentricity between core and coating. As such, commercial optical fibers do not lend themselves to precision alignment. Misalignment between fibers or fiber and optical component (i.e., photodetector) is the primarily source of light energy loss.
Optical fiber systems typically require a hermetic seal at fiberxe2x80x94fiber connections, fiber-component connections, and along the length of a fiber to prevent moisture and other contaminates from degrading the optical pathway. Commercially available coated fibers are porous and therefore fail to provide a hermetic seal sufficient to exploit component lifetime. Furthermore, porous coatings reduce adherence between coating and fiber thereby weakening connections.
Coated optical fibers are typically soldered to other components thereby providing a continuous pathway. The pull strength of the coated fiber at such connections is critical to the integrity of the pathway. Currently, coating design and fabrication methods limit pull strength to approximately 1.6 pounds as verified by quality assurance tests known within the art. Coating methods may also further weaken the fiber by creating micro-cracks within the fiber structure.
Various methods are known within the art to coat an optical fiber with a metal layer, see Kruishoop et al. (U.S. Pat. No. 4,609,437), Cholewa et al. (U.S. Pat. No. 5,100,507), Filas et al. (U.S. Pat. No. 5,380,559), and Dunn et al. (U.S. Pat. No. 5,970,194). The related arts have sought to minimize dependence on sputtering and to develop replacement methods.
Kruishoop et al., issued Sep. 2, 1986, describes a two-step method to form a metal coating onto a synthetic resin cladding along an optical fiber. A thin conductive layer is first applied by reducing a metal salt onto the cladding and thereafter forming a thin metal layer by electroplating. Kruishoop et al. explicitly excludes sputtering methods for applying the conductive layer since such methods produce thermal energy sufficient to damage the underlying structure.
Cholewa et al., issued Mar. 31, 1992, describes a method for processing an optical fiber comprised of an integral lens and a metallized outer coating. Metallization is achieved via sputtering. However, Cholewa et al. does not address the thermal heating problem and damage inherent to sputtering as identified by Kruishoop et al.
Filas et al., issued Jan. 10, 1995, describes an electroless method for depositing nickel and gold coatings onto optical fibers using aqueous chemistry. The Filas et al. method was developed since sputtering is not only expensive, but also produces a non-uniform coating and tends to weaken the fiber.
Dunn et al., issued Oct. 19, 1999, describes a method wherein a limited mid-section of an optical fiber is metallized via sputtering or evaporation. Dunn et al. does not address the problems inherent to sputtering as identified by Kruishoop et al. and later by Filas et al.
Planar sputtering methods are known within the art. Planar sputtering deposits a thin film coating onto a fiber as it rotates relative to a uni-directional coating source. Both stationary and moving fibers are coated with this technique.
Planar sputtering methods are complex, inefficient, and fail to provide the uniformity and quality required for many optical fiber applications. Planar sputtering requires mechanically complicated precision rotation means to adjust the fiber with respect to the planar source. Such rotating systems cannot ensure sufficient thickness uniformity for accurate fiber alignment. Planar sputtering is inefficient in that only a small portion of the metal ejected from the target is deposited onto the fiber thereby making its use costly. Planar sputtering subjects the fiber to asymmetric overheating across the cross section of the fiber thereby promoting microcracks within the cladding and reducing the quality of the coating. Furthermore, planar sputtering yields a porous coating reducing hermeticity and adherence.
Kumar describes a cylindrical magnetron for applying a circumferential coating, see U.S. Pat. Nos. 5,178,743 issued Jan. 12, 1993 and 5,317,006 issued May 31, 1994. While cylindrical magnetron inventions are disclosed, methods for depositing precise, dense, and adherent coatings without damaging an optical fiber are neither described nor claimed.
It is therefore an object of the present invention to avoid the disadvantages of the related art. More particularly, it is an object of the invention to provide a coated optical fiber with minimal dimensional variability thereby facilitating rapid alignment and assembly of such fibers within an optical system. It is an object of the invention to provide a dense, low-porosity coating onto a fiber substrate. It is an object of the invention to provide improved adherence between coating and fiber substrate. It is also an object of the invention to provide a controlled method for depositing a coating onto a fiber without damaging the fiber. It is an object of the invention to provide a coated fiber with greater pull strength. It is an object of the invention to provide a coating method facilitating the simultaneous application of one or more coatings onto a plurality of fibers. Furthermore, it is an object of the invention to provide a coating method that facilitates the application of several independent layers within a single vacuum chamber without breaking the vacuum.
The present invention provides a controlled method for the application of an improved coating onto an optical fiber while avoiding damage to the fiber structure. The improved coating is applied via sputtering within a cylindrical magnetron.
The claimed deposition method includes generating a plasma cloud composed of dimensionally similar sputtered particles that adhere to an optical fiber and form a uniform, adherent, low-porosity coating, monitoring at least one environmental parameter within the vacuum chamber during deposition, and adjusting the deposition step to avoid one or more conditions that promote fiber damage. Monitoring and adjusting steps are either manually controlled or automated. Environmental conditions include such examples as temperature, pressure, and gas composition, each indicative of the onset or progression of fiber damage.
An optional cleaving step is provided. In one embodiment, a portion of the fiber end is removed to expose an optically clear core. In another embodiment, the fiber mid-section is cleaved yielding two fiber ends, each having an optically clear core.
The deposition method is applicable to fiber ends, fiber mid-sections, as well as along the length of the optical fiber. One or more optical fibers may be simultaneously coated in one or more cylindrical magnetrons thereby increasing production yield.
The improved coating includes single and multiple layer configurations. In one embodiment, at least one layer is composed of a thermal barrier material applied directly onto an optical fiber, a metal layer, or another thermal barrier material. In yet another embodiment, the layers are composed of commercially pure metals.
Both coating embodiments facilitate a stronger optical fiber in conventional pull test arrangements. Thermal barrier coatings are inherently stronger due to mechanical properties of the materials and improved adherence between such materials and fiber, as well as between such materials and other layer materials. Metal-based coatings are inherently stronger because either the coating compressively constrains the fiber or the coating closes microcracks within the fiber prior to or as a result of the sputtering process.